Parity with iOS SDK with regards to providing the ability to set a custom debug token for AppCheck's Debug Provider.

[joncobber]

What feature would you like to see?

I would like to be able to set a debug token on the AppCheck Debug Provider either directly, or indirectly with an environment variable, in the same manner the iOS SDK affords (https://firebase.google.com/docs/app-check/ios/debug-provider).

It would be nice if both platforms had the same degree of flexibility.

How would you use it?

I would use it a few ways:

1. For testing, I would just be able to set a value in CI and I wouldn't have to wrap tests in blocks executed by the DebugAppCheckTestHelper class.

2. I would create and rotate a known set of debug tokens in my firebase projects for my internal releases of my application and allow developers to set this value in their local builds so that manual token management via log output and the firebase console would be avoided.

[google-oss-bot]

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